Motor control circuit and three-phase generator with stationary components

ABSTRACT

An electrical three-phase generator and motor control circuit. The generator is made of three alike wave-generating units in cascade each generating a respective trapezoidal wave corresponding to a respective phase of the generator. Each unit comprises a small detector in series with a control circuit varying the amplitude of the wave proportionately to a control voltage representative of and corresponding to variations of voltage from voltage sources and applied to a motor and an integrating circuit consisting of an integrating amplifier and an integrating capacitor and a double limiter in parallel therewith. The control voltage is developed by a voltage divider across the motor control circuit.

United States Patent 1 Jensen 1 Oct. 23, 1973 [54] MOTOR CONTROL CIRCUITAND 3,444,394 5/1969 Colvson 307/261 THREEJJHASE GENERATOR WITH3,089,992 5/1963 Seney 321/61 UX STATIONARY COMPONENTS Inventor: ArneJensen, Havnbjerg Als,

Denmark Assignee: Danfoss A/S, Nordbog, Denmark Filed: Dec. 3, 1970Appl. No.1 94,934

Related U.S. Application Data Division of Ser. No. 755,190, Aug. 26,1968, Pat. No. 3,597,638.

Primary Examiner-J. D. Miller Assistant ExaminerHarvey FendelmanAtt0rneyWayne B. Easton 57 ABSTRACT An electrical three-phase generatorand motor control circuit. The generator is made of three alikewavegenerating units in cascade each generating a respective trapezoidalwave corresponding to a respective phase of the generator. Each unitcomprises a small detector in series with a control circuit varying the[52] US. Cl 318/227, 318/231, 321/9 A, mpli ude of the waveproportionately to a control 321/45, 321/D1G. 1, 307/228, 307/261voltage representative of and corresponding to varia- [51] Int. Cl. H02p5/28 tions of vol ge from voltage sources and applied to a [58] Field ofSearch 318/171, 227, 231; m r an an int gra ing circuit consisting of aninte- 321/45 R, 9 A, 61, 66, 69 R, DIG. l grating amplifier and anintegrating capacitor and a double limiter in parallel therewith. Thecontrol volt- [56] References Cited age is developed by a voltagedivider across the motor UNITED STATES PATENTS control clrcult'3,331,003 7/1967 King 318/231 4 Claims, 7 Drawing Figures 3,355,64711/1967 Braus.... 3,125,694 3/1964 Palthe 307/228 23 B 3 PH R I9 0. c.(PULSATING) J rmeesa '5 POWER S OURCES 5 l SWITCH MOTOR I8 P' MOTORCONTROL CIRCUIT AND THREE-PHASE GENERATOR WITH STATIONARY COMPONENTSThis is a divisional application of application Ser. No. 755,190 filedAug. 26, 1968 now U.S. Pat. No. 3,597,638.

This invention relates generally to three-phase generators and moreparticularly to an electrical three-phase generator with stationarycomponents.

Single-phase generators with stationary components for generating asinusoidal voltage, are known. However, the employment of three suchknown sinusoidal voltage generators in combination or assembly to form athree-phase system presents difficulties. While it is possible to varythe frequency in known sinusoidal voltage generators, any variation ofthe frequency involves a transient period which increases the responsetime of the assembly.

A principal object of the present invention has for its object toprovide a three-phase generator consisting of stationary components andnevertheless producing, as nearly as possible, the voltagecharacteristic of a rotary three-phase generator (sinusoidal,instantaneous combined voltages equal to zero) and enabling a simple,rapid variation of frequency, and also within the range of industrialalternating voltages from to 100 Hz or cycles per second.

This object is achieved by the invention in that the half-waves of theindividual phases are trapezoidal waveshapes and the three straightsections of the trapezium each have a duration of 60 electrical degrees.

In this way advantage is taken of the fact that trapezoidal half-waves,especially in the lower frequency region, can be generated more easilythan sinusoidal waves when using stationary components. The trapezoidalcharacteristic claimed by the invention has the advantage over any othertrapezoidal waveshape that, in a three-phase system, the resultinginstantaneous summation voltage is always zero. The waveshape is sonearly sinusoidal that the third harmonic (as well as any other harmonicdivisible by 3) is eliminated and merely the inconsiderable 5th, 7th,11th etc., harmonics of small amplitude need to be taken into account(for the nth harmonic, with the factor n). The amplitude of the relevantfundamental frequency is larger by 5 percent than the amplitude of thetrapezoidal characteristic so that this 5 percent is available for thecompensation of diode, filter etc. losses if a sinusoidal wave is to berepresented.

This trapezoidal characteristic in which the three straight sectionseach have the same duration allows frequency to be varied very easily.All that has to be done is to vary the slope of the edges whilemaintaining the amplitude of the trapezoidal half-wave.

In a preferred circuit arrangement of the three-phase generator threewave-generating units are connected in series or cascade in a loop. Eachof the three units, in dependence on the sign of the trapezoidal wave ofthe preceding unit, generates a square wave of constant but preferablycontrollable amplitude in phase therewith and then integrates thissquare wave and clips the integrated signal on either side to a constantamplitude. This results in a closed system in which the first unitdrives the second unit, the second unit drives the third unit, and thethird unit drives the first unit. In this way, not only the correctphase position of the several halfwaves becomes fixed, but also theduration of all the straight portions of the trapeziums or trapezoidalwaveshapes. The amplitude of the square wave determines frequencybecause it controls the slope of the trapezium waveforms duringintegration. According to a modified concept it would also be possibleto use, instead of a square wave in phase with the signal, aphase-displaced square wave, provided that an even number of such phasereversals occurs within the loop. In order to alter the frequency, thecontrollable amplitude of the square wave must be set to the same valuein all units in a common operation. Since this is done simultaneously inthe relevant locations of the system, transient phenomena do not occur.

In a practical embodiment of the invention each unit comprises, inseries, a null detector with a preferably high amplification and aninput signal in phase with the output signal, as well as an integratingamplifier the integrating capacitor of which is in parallel with adoubleended or double limiter circuit. Between the null detector and theintegrating amplifier may be provided a double-ended limiter for theamplitude of the square wave which should however be adjustable by meansof a direct current voltage. In this way, the frequency of the generatormay be changed simply by varying a direct current voltage.

It is known that when controlling induction motors the torque remainsconstant if the frequency and motor voltage vary proportionally to oneanother. Up to now it was very difficult to fulfill this condition. Witha generator according to the invention, on the other hand, it issufficient to vary the amplitude of the square wave in proportion withmotor voltage because the frequency is proportional to the amplitude.Since the amplitude, in turn, can be varied in the invention by means ofa direct current voltage, all that is necessary is for this directcurrent voltage to be proportional to the motor voltage.

For example, an arrangement can be made in which an asynchronous motordriven with a pulsating direct current is controlled by a direct voltagewhich is applied to a square wave amplitude-limiting circuit andproportional to the motor supply voltage and the generator frequencycontrols the periodic on-off switching or change-over of the supplyvoltage. By way of modification, the direct voltage applied to thelimiter may proportionally control frequency as well as supply voltage.

The applications of the generator of the invention are numerous. It canbe employed wherever a three-phase system, especially avariable-frequency system, is needed. It primarily-serves for motorcontrol in which case the three-phase voltage is directly applied to themotor or is used for controlling the motor supply voltage. It is notessential for the trapezoidal voltages to be derived from all threeoutputs. In many cases only one phase will be used, for example when asingle-phase alternating voltage with easily variable frequency isdesired.

Other features and advantages of the three-phase generator in accordancewith the present invention will be better understood as described in thefollowing specification and appended claims, in conjunction with thefollowing drawings, in which:

FIG. 1, is a block diagram of a three-phase generator according to theinvention;

FIG. 2, is a diagram of the phase voltages generated by the generator ofFIG. 1;

FIG. 3, is a block diagram of a wave-generating unit in the three-phasegenerator of FIG. 1;

FIG. 4, is a diagram of the signal voltage characteristics or waveforms,plotted against time, produced .by the unit of FIG. 3;

FIG. 5, is a circuit diagram of an embodiment of a unit according toFIG. 3;

FIG. 6, is a control circuit for an electric motor employing a generatorof the invention, and

FIG. 7, is another control circuit employing the generator of theinvention. v

According to the drawing FIG. 1 shows a three-phase generator of theinvention which comprises three identical wave-generating units 1, 2,3at the outputs of which appear output phases R, S and T in reversedsequence. The output of the preceding unit always consti tutes the inputof the following unit, and thus also the output of unit 3 constitutesthe input of unit 1. A control signal i B can be simultaneously appliedto all three units and is preferably a direct-voltage pulse. FIG. 2illustrates the shape and position of the phase voltages R, S, T thusproduced.

Each of these waveform-generating units is constructed as showndiagrammatically in FIG. 3 in which only one of the units isillustrated. An input signal a, i.e., the'output signal of the precedingunit, is applied to a null detector 4 providing high amplification. Theresulting output signal b is asquare wave in phase with the initialsignal which is applied ,to a control circuit 5 in which the amplitudeof the square wave b can be varied by means of the control signal iB. Anoutput signal c thus produced by the control circuit 5 is applied to anintegrator 6 comprising an integrating resistor 7 in series with aparallel circuit consisting of an amplifier 8, an integrating capacitor9 and two voltage limiters 10, 11 preferably constructed as Zenerdiodes, seriesconnected back to back. The output signal d of theparallel circuit will then be a trapezoidal wave of the same shape asthe input signal a, but phase-displaced by 240.

FIG. 4 illustrates the characteristics or waveshapes of the signals a-d,plotted against time on a baseline. Each of the trapezoidal half-wavescomprises three straight sections, i.e., a rising slope or leading edge12, the constant amplitude 13 and a falling slope or trailing edge 14.Each of these three sections extends over an electrical angle of 60. Thegiven duration of' these 60 sections or periods depends, of course, onthe frequency of the signal and decreases as frequency rises. The nulldetector 4 merely ascertains the sign of signal a, so that a square waveb of standard amplitude is the result. This amplitude may be varied inthe control circuit 5 by means of the control signal B, as can be seenby regarding signal 0. As the square wave is integrated in the amplifier6, the falling slope l4 and the rising slope 12 of a trapezoidal wa'veare produced. When the threshold of the voltage limiter 10, 11 isattained, integration stops and the amplitude 13 of the trapezoidal waveis kept at a constant value for the duration of 60 electrical degrees.Then the square wave c changes its sign and the cycle commences again inthe opposite direction. If this process is repeated for all threewaveforming units, then the trapezoidal voltage applied to the firstunit will just be at the proper level to drive the first unit.

If the control signal B in the control unit 5 is used to vary theamplitude of signal 0, the steepness of the slopes l2 and 14 is changed.Due to the loop arrangement of the three wave-forming units and to thesimultaneous influence on all signals c the condition is fulfilled thatthe sections l2, l3 and 14 always retain the same duration, the systemreverts to another frequency attained immediately after a change ofsignal 8, without any transient phenomena.

FIG. 5 is a circuit diagram for one such wave-forming unit. The unit issupplied with +22 volts and 22 volts as illustrated and comprises aninput for the signal a, an output for the signal d and a control inputfor voltages +B and B. The signal a is applied to an amplifying zerolevel detector comprising two galvanically coupled complementarytransistors Trl and Tr2. The signal a is applied to the'base of thetransistor Trl through a resistor R1. The collector of this transistorTrl is connected through a resistor R2 to the base of the followingtransistor Tr2. In order to safely establish zero level, thebase-emitter path of the input transistor is connected through a diodeE1 to a reference zero potential, in this case the mid-point of acurrent supply for the generator, not shown in FIG. 5 but shown in FIGS.6 and 7. This circuit arrangement also serves for thermal stabilization.A capacitor C1 filters out highfrequency oscillations. The output signalfrom the second transistor Tr2 is the square wave b.

The control circuit 5 in this case is a simple limiter. Between twoseries-connected resistors R3 and R4 two oppositely connected diodes E2and E3 are branched off to which the variable control voltage B and +8is applied. These diodes begin to conduct as soon as the amplitude ofthe square wave b, the output of the null detector, exceeds the level ofthe control voltages :B. The square wave 0 at the limiter output istherefore a signal of reduced amplitude.

The resistor R4 is also the input resistor of the integrator 6. Theintegrator comprises a differential amplifier 8 the input section ofwhich consists of a transistor Tr3 whereas its output section comprisestwo complementary transistors Tr4 and TrS. The latter transistors areinterconnected through the resistor R6.'The base of the transistor Tr4is connected to the common reference level of the system and a diode E4is provided in the emitter circuit of the output transistor Tr5. Fromthe collector of the output transistor TrS the signal is fed backthrough an integrating capacitor C2 to the base of the transistor Tr3.The Zener diodes E21 and E22, series-connected back to back, correspondto the diodes 10, 11 and are in parallel with the integrating capacitorC2 which corresponds to the integrating capacitor C9. This limits thevoltage across the integrating capacitor C2 and produces the sections 13of each trapezoidal half-wave.

In the circuit arrangement of FIG. 6 an asynchronous motor 15 issupplied alternately with pulsating direct current from a positivevoltage source 16 and anegative voltage source 17.The voltage of thesetwo sources is controlled by a common unit 18 illustrateddiagrammatically. Switching of the voltage is effected by means of twoswitches 19, 20, alternately operated by a trigger 21. This trigger iscontrolled by a generator 22 such as proposed by the present invention,only one phase of this generator is illustrated being used. The twovoltage sources are each shunted by one potential divider comprisingresistors 23, 24 and 25, 26 respectively. The control voltage +8 and Bfor the generator 22 is tapped from the junctions of the potentialdividers as illustrated. The control voltage B varies proportionallywith the motor supply voltage. Since the frequency of the output signalof generator 22 also varies proportionally with the level of the controlvoltage B, the switching frequency of switches 19, 20 variesproportionally with the motor supply voltage, so that the motormaintains a constant torque at every speed.

In FIG. 7 the same reference symbols as in FIG. 6 have been used foridentical components. The essential difference is that the controlsignal B is here used not only for controlling the switching frequencyof switches 19, 20, but also for controlling the supply voltage. Forthis purpose, a signal h proportional to the control voltage B, isderived from the generator 22 which influences two controllable voltagesources 27, 28 in the appropriate manner.

While preferred embodiments of the invention have been shown anddescribed it will be understood that many modifications and changes canbe made within the true spirit and scope of the invention.

What I claim and desire to be secured by Letters Patent is:

1. In combination in a motor control circuit for an induction motor, aninduction motor, voltage sources energizing said motor, means developinga control voltage varying proportionately to variations of the voltagefrom said sources applied to said motor, means comprising a three-phasegenerator under control of said control signal generating three waveseach comprising positive and negative half-waves each having atrapezoidal waveform, and means applying the half-waves to said motor tocontrol its torque and maintain it substantially constant regardless ofvariation of the voltage from said sources, said three-phase generatorincluding (1) three wave-generating units in cascade with each unitincluding a null detector, (2) a control circuit receptive of saidcontrol signal and in series with said null detector, and (3) anintegrating circuit receptive of the output of said control circuit,said trapezoidal waveform comprising three sections with each saidsection having a duration of 60 electrical degrees.

2. The combination according to claim 1, in which said integratingcircuit comprises an integrating amplifier, an integrating capacitor inparallel with said amplifier, and a double limiter across said amplifierin parallel with said capacitor.

3. The combination according to claim 2, in which said double limitercomprises two zener diodes connected back to back.

4. The combination according to claim 1 in which said null detectorcomprises two series transistors, one of said transistors having anoutput collector, said amplifier comprising an input transistor having abase and an output transistor, two resistors connecting said outputcollector to said base of said input transistor, two parallel diodesconnected in opposite polarity at junctions between said resistors, saiddouble limiter comprising two zener diodes connected back to back acrosssaid amplifier.

1. In combination in a motor control circuit for an induction motor, aninduction motor, voltage sources energizing said motor, means developinga control voltage varying proportionately to variations of the voltagefrom said sources applied to said motor, means comprising a three-phasegenerator under control of said control signal generating three waveseach comprising positive and negative half-waves each having atrapezoidal waveform, and means applying the half-waves to said motor tocontrol its torque and maintain it substantially constant regardless ofvariation of the voltage from said sources, said three-phase generatorincluding (1) three wave-generating units in cascade with each unitincluding a null detector, (2) a control circuit receptive of saidcontrol signal and in series with said null detector, and (3) anintegrating circuit receptive of the output of said control circuit,said trapezoidal waveform comprising three sections with each saidsection having a duration of 60 electrical degrees.
 2. The combinationaccording to claim 1, in which said integrating circuit comprises anintegrating amplifier, an integrating capacitor in parallel with saidamplifier, and a double limiter across said amplifier in parallel withsaid capacitor.
 3. The combination according to claim 2, in which saiddouble limiter comprises two zener diodes connected back to back.
 4. Thecombination according to claim 1 in which said null detector comprisestwo series transistors, one of said transistors having an outputcollector, said amplifier comprising an input transistor having a baseand an output transistor, two resistors connecting said output collectorto said base of said input transistor, two parallel diodes connected inopposite polarity at junctions between said resistors, said doublelimiter comprising two zener diodes connected back to back across saidamplifier.